It creates a false clear channel assessment. When the laptop goes to transmit he checks to see if the air is clear. Because that node is hidden from him he may start talking when the other node is transmitting.

By definition, any client associated to an AP is within range of the AP - of course. By using the RTS/CTS or CTS-self on the AP, every client is notified of the up-coming transmission, and the effects of Hidden Node are mitigated.